Friday, March 23, 2012

Brittle stars (class Ophiuroidea) are marine animals typically with five thin, segmented arms extending from a central disc. "Ophiuroidea" means "snake-like", refering to the animals' long and flexible arms, and hence they are also often called serpent stars.

When disturbed, brittle stars cast off parts of their arms (thus the common name "brittle star"), not unlike a lizard's tail, to distract any possible predator. Like other members of the phylum Echnodermata, they are able to regenerate the lost arms.

The echinoderms are often called the spiny-skinned animals (phylum Echinodermata - "echino" roughly means "spiny"; "derma" roughly means "skin"). Examples of other echinoderms include sea stars, sea cucumbers, and sea urchins. Like other echinoderms, brittle stars have a five-part body plan with radial symmetry (i.e. pentaradial symmetry), at least in some stage of life. In other words, you can divide a brittle star into 5 equal parts.

Echinoderms are brainless, but despite that, they can still perform their daily functions - they can move, they can eat, they can shit, and they can reproduce - all these without a brain! Also, instead of blood vessels, echinoderms have a water vascular system. This system is essentially a network of water-filled vessels used for internal transportation of oxygen, food and waste. By adjusting the water pressure, the brittle star is able extend and retract its arms to move and feed.

Brittle stars mostly forage at night, and hide in crevices or under rocks in the day. Some lives commensally with other organisms - it is believed that living with other animals may give them better access to food and shelter, but their presence does not affect the host animal.

Brittle stars have a mouth on the underside but no anus. They mostly feed on tiny organic particles (detritus) collected with their tube feet on the underside of their arms, though some species may feed on smaller animals.

Some species burrow into the sand in the day, emerging only at night. More than 30 species of brittle stars have been seen in Singapore.

Order Phrynophiurida

Members of this order are usually called basket stars. They are a special group of brittle stars with arms branching into smaller arms, forming a network branches. The arms are usually spread out like a basket to catch plankton in the water column.

Several basket stars (suborder Euryalina) also live with sea fans, such as this tiny unidentified species about 2cm wide found during a dredging trip.

Bigger basket stars, such as this unidentified species, will either cling to the substrate or other organisms to catch its food.

The above is another basket star found on a sea fan in the intertidal area.

Order Ophiurida

Most of the brittle stars are from this order. They are characterised by unbranched arms and scales covering the upper side of the disc.

Family Ophiactidae

Brittle stars from this family generally have discs covered with spines and granules, thought they usually do not conceal the scales. They have short, erect spines. They are distinguished by a rectangular, block-like oral papilla (protruding structure at the mouth) at the tip of each jaw.

Some brittle stars form this family, such as the Ophiactis species above, live in sponges or other animals.

Ophiactis savignyi, for example, has been found living with sponges, feather stars and sea stars. Sometimes they can also be found living on their own, either hiding under rocks and or in crevices. They are small, about 1-2cm wide including the arms, and green in colour.

Family Ophiotrichidae

Brittle stars from this family have well-developed scales. Their arms are
flexible in all directions, and come with delicate translucent, thorny spines. They lack oral papillae, but instead have papillae on their teeth.

Ophiomaza cacaotica from this family can be found living on feather stars. It has distinctive black (or dark purple) and white patterns on the disc, and the arms has a pair of black lines running down the sides. This specimen which I saw was about 10cm wide, including the arms.

The colour and patterns of Ophiomaza cacaotica may vary. The above is a specimen that is mostly white.

The above Ophiomaza cacaotica is mostly black instead.

Ophiothela species are usually also commensals, and have also been seen on sea pens, soft corals and sea fans. These small commensals are less than 1cm across, and come with colourful bands on their arms.

This Ophiothela danae lives on sea fans, clinging tightly to the branches.

The above is a closer look at the Ophiothela danae on a sea fan.

The Jellyfish Brittle Stars (Ophiocnemis marmorata) is sometimes seen on large jellyfish stranded on our shores.

Here is a closer look at the Jellyfish Brittle Star. This brittle star are commensals on large jellyfish. The exact relationship is not known, though scientists suspect that they live on jellyfish for protection and also for food - they feed on the leftover food of their hosts. It was also suggested that only the small ones live on jellyfish, while the bigger ones would eventually drop off and hence get dispersed.

Ophiothrix plana is seen once at Semakau. It is dark green in colour, with long translucent spines on the arms.

Ophiothrix lineocaerulea emerges from its hiding place and forages in the seagrass meadows at night. It has a characteristic colour pattern of two dark parallel lines along the top side of each arm. Bluish bands can be seen on the arms as well.

Ophiothrix melanosticta has pretty red spots on its arms. It has dense bottlebrush-like spines on its arms. It has been seen on soft corals sometimes.

The closely related Macrophiothrix longipeda is more often found on reef flats. It is usually grey with thin black bands running down the arms. A pair of thin parallel black lines run along the top sides of the arms.

Family Amphiuridae

Brittle stars from this family are usually small and fragile. The jaws come with a pair of papillae at the tip, and one or more on each side.

The small Amphipholis squamata is usually greyish or bluish white in colour, and about 2cm wide. It is usually found under rocks and shells.

Family Ophiodermatidae

Brittle stars of this family have arms projecting from, and well fused to, the edge of the disc. The main disc is usually covered in dense coating of granules, and the arm spines are short and hardly noticeable without a microscope.

This Ophiodyscrita acosmeta has dark brown bands running along its arms on a pale brown background.

Many of our brittle star species are not yet identified, and like most other marine organisms, the ophiuroids in Singapore are threatened by loss of habitats and collection. Here are a few of the brittle stars which I could not identify.

This tiny brittle star is about 1cm wide, and lives on a sea apple.

This unidentified brittle star has bright yellow bands on its arms. It was found among seagrasses growing on a sandy substrate.

This unidentified brittle star with bluish bands on its arms was found among seagrasses on a sandy-muddy substrate.

Feathers stars or crinoids (class Crinoidea) are marine animals with five or more feather-like arms. Each arm carries comb-like structures called pinnules, comprising rows of tube feet used for capturing plankton. On the underside, most feather stars have claw-like arms called cirri to hold on to the substrate. Their skeleton is made of numerous calcareous plates.

Like other echinoderms, feather stars have a five-part body plan with radial symmetry (i.e. pentaradial symmetry), at least in some stage of life. In other words, you can divide a feather star into 5 equal parts. Also, they are able to regenerate lost body parts - such as an arm bitten off by a predator. Echinoderms are brainless, but despite that, they can still perform their daily functions - they can move, they can eat, they can shit, and they can reproduce - all these without a brain! Also, instead of blood vessels, echinoderms have a water vascular system. This system is essentially a network of water-filled vessels used for internal transportation of oxygen, food and waste. By adjusting the water pressure, the feather star is able extend and retract its arms for feeding. They can even swim by flapping their arms.

Many feather stars feed in the night hide in crevices in the day, though a number of them can be seen in the day too. 18 species of crinoids have been recorded from Singapore waters, but they are just too difficult to identify for the lay person.

Here are some feather stars that I have photographed, mostly unidentified.

Red feather star (Himerometra sp.).

Comatula purpurea?

Stephanometra sp.?

Stephanometra sp.?

Stephanometra sp.?

Like most other marine organisms, the crinoids in Singapore are threatened by loss of habitats and collection.

Sea urchins (class Echinoidea) are animals with a round, rigid skeleton (test) made of interlocking calcite plates. The test is hollow inside, containing its various organs, and covered with lots of spines on the outside. They belong to a bigger group of animals called the spiny-skinned animals (phylum Echinodermata - "echino" roughly means "spiny"; "derma" roughly means "skin"). Examples of other echinoderms include sea stars, sea cucumbers, and feather stars.

The spines can come in various forms - long or short, smooth or rough, sharp or blunt. They not only help the sea urchin to move, but deter predators as well. Some sea urchins are venomous, and hence it's a good habit not to pick up any of them. The venom may be delivered by the spines or tiny stalk-like structures with biting jaws called "pedicellariae".

Like other echinoderms, sea urchins generally have a five-part body plan with radial symmetry (i.e. pentaradial symmetry), at least in some stage of life. In other words, you can divide a sea urchin into 5 equal parts. Also, they are able to regenerate lost body parts - such as spines lost to predators. Echinoderms are brainless, but despite that, they can still perform their daily functions - they can move, they can eat, they can shit, and they can reproduce - all these without a brain! Also, instead of blood vessels, echinoderms have a water vascular system. This system is essentially a network of water-filled vessels used for internal transportation of oxygen, food and waste.

The mouth of a sea urchin is on its underside, comprising five elongated vertical jaws held together in a structure known as the Aristotle’s lantern. Sea urchins generally feed on algae and seagrasses, though some may scavenge. The anus is on the top side. They have separate sexes, and usually reproduce by broadcasting their eggs and sperms into the water.

There are two main groups of sea urchins: the regular sea urchins with spherical tests; and irregular sea urchins with more flattened tests that are bilaterally symmetrical. The latter generally lacks the Aristotle’s lantern as well. Interestingly, the regular sea urchins are usually found in seagrass meadows and coral reefs, while the irregular sea urchins are typically burrowers in sandy substrates. In Singapore, more than 20 species of sea urchins have been recorded.

Here are the sea urchins found in Singapore that I managed to photograph.Order Diadematoida

Members of this order are regular sea urchins with hollow spines, many of which contains venom. Many have long spines.

The Long-spined Sea Urchin (Diadema setosum) is a regular sea urchin commonly seen on our reefs. It has long, needle-like spines which break easily upon contact. These spines are hollow, containing a mild venom, and broken spines are very difficult to remove when they penetrate the victim's skin of the victim, causing pain and discomfort. This sea urchin has an orange anal cone and five equally spaced white spots on the upper side. The test can be up to 8cm wide.Order Camarodonta

Members of this order are regular sea urchins, mostly with shorter spines. Their lanterns come with keeled teeth, and they usually have enlarged plates.

The Black Sea Urchin (Temnopleurus toreumaticus) is seasonally abundant on Singapore's northern shores, and at times hundreds, if not thousands, can be found along a shore among seagrasses.

In Latin, "toreuma" means "carved or embossed work", possibly refering to the numerous intricately arranged bumps (where the spines and tube feet extend from) on the small test (up to 4cm wide).

The White Sea Urchin (Salmacis sp.), also seasonally abundant, usually occurs in seagrass meadows. They are often found camouflaged with dead shells, rock fragments, seaweed or seagrass. While it can occur in similar habitats as the previous sea urchin, it is usually much larger, with the test reaching almost 10cm wide.

While they are mostly whitish in colour, individuals with brown, purple or pink spines have been seen too. In the water, translucent tube feet can be seen extending out, which sticks to surface and hence help the sea urchin to move around and camouflage.Order Cidaroida

Members of this order have thick, blunt spines on their tests. They somewhat resemble
pencil-holders with "pencils" poking out, and hence the common name pencil sea urchins.

The Thorny Pencil Sea Urchins (Prionocidaris spp.) are seasonally abundant. Their thick spines often come with darker bands. Bits and pieces of seaweed and seagrass are often found entangled among the spines.

Two species have been seen in Singapore: one with thicker and shorter spines, and another with thinner, longer and sharper spines. One of the species also has purple or pinkish spots at the base of the main spines. The two sea urchins above should be Prionocidaris bispinosa, since they lack the spots. Some of the bigger specimens may have tests reaching width of more than 8cm.Order Spatangoida

The sea urchins from this order are usually called heart urchins. They are irregular sea urchins with oval or heart-shaped
tests. Unlike other sea urchins, they have a definite front and rear end.
They often burrows in the sand and hence, they are not often spotted -
there could be many of them under the sand that you are standing on!
They feed on tiny organic particles (detritus).

Occasionally, they can be seen on the sand surface or on pebbles near sand, moving around with their short spines on the underside. Three species have been recorded from Singapore.

The Elongate Heart Urchin (Lovenia elongata) is heart-shaped, usually reddish or pinkish in colour, and about 3-5cm long. It usually comes with a few longer spines pointing backwards.

The Ovate Heart Urchin (Maretia ovata) is larger, about 5-8cm long, and whitish in colour. It appears more oval than heart-shaped, and lacks the special longer spines found in the former species.

The Longspine Heart Urchin (Maretia planulata) is the smallest heart urchin recorded from Singapore. It is about 2-3cm long with brownish or reddish flower-like pattern on the top side. It has many long spines pointing backwards.Order Clypeasteroida

Sea urchins from this order are often called sand dollars. They are also irregular sea urchins, and as their name suggests, they are mostly round shaped like coins, and live in the sand bank! They have a flat test covered with short, hair-like spines which helps them to move around and burrow. On the top side of sand dollars are five rows of pores in a flower-like pattern, and it is through these patterns that their tube feet poke out to breathe. These animals probably feed on algae and detritus.

The Placenta Sand Dollar (Arachnoides placenta) is the commonest sand dollar, and can occur in the hundreds in some areas. It is usually found on sandy shores with very fine sand. it can grow to about 10cm across.

More rarely seen is the Keyhole Sand Dollar (Echinodiscus truncatus) which has 2 distinctive slots (lunules) on the test. The exact function is still not fully understood, though scientist believe that they probably allow water to flow through and help prevent the sand dollar from being flipped over by strong currents. It can also grow to about 10cm across.

The Lesueur's Sand Dollar (Peronella lesueuri) is sometimes seen on Singapore's northern shores. It is pink in colour, and is usually found among seagrasses, sometimes just beneath the sand surface. The biggest I have seen is about 8cm across.

The Depressed Sand Dollar (Laganum depressum) is sometimes seen, and usually several to many of them are seen together. They are usually light brown or pinkish brown in colour. I noticed it is often found in areas with some freshwater runoffs though. It is about 3-4cm across.

Like most other marine organisms, the sea urchins in Singapore are threatened by loss of habitats and collection.